Messenger No.
41 (September 1985)

ADS BibCode:1985Msngr..41....1WAuthor(s)/Affiliation(s):West, R. M.AA(ESO)Abstract:While Periodic Comet Halley rapidly approaches the Sun, preparations are being made in many places to observe this distinguished celestial visitor. During most of the months of June and July 1985, Halley was "behind" the Sun and could not be observed. From about July 15, attempts to obtain images of Halley were made in various places, and it now appears that the first confirmed sighting was made at the European Southern Observatory on July 19.

ADS BibCode:1985Msngr..41....7BAuthor(s)/Affiliation(s):Baade, D.; Ponz, D.; di Serego Alighieri, S.AA(The Space Telescape European Coordinating Facility, European Southern Observatory) AB(European Southern Observatory) AC(The Space Telescape European Coordinating Facility, European Southern Observatory,Affiliated to the Astrophysics Division. Space Science Department,European Space Agency,On leave from Osservatorio Astronomico, Padova)Abstract:It is now about six months since ESA's Photon Counting Detector (PCD), the ground-based counterpart to the FaintObject Camera (FOG) to be flown with the Hubble Space Telescope, was put into operation at la Silla and made available to Visiting Astronomers. There it proved to be very reliable and, also thanks to good weather conditions, was very productive. By now, quite a few readers of the Messenger will be busy analysing their own PCD observations.References:Boksenberg, A.: 1972, in Proc. ESO/CERN Workshop "AuxiliaryInstrumentation tor Large Telescopes", eds. S. Laustsen and A.Reiz, Geneva, p. 295.di Serego Alighieri, S., Perryman, M.A. C., Macchelto, F.: 1985,Asuon.Asuophys. 149, 179.

ADS BibCode:1985Msngr..41...11WAuthor(s)/Affiliation(s):Wampler, E. J.AA(ESO)Abstract:In the early days of the IDS development crude tests of the system indicated that its response was approximately linear with intensity (McNall, Robinson and Wampler, Pub. A. S. P. 82,488,1970; C. M. GaskeIl; J.A. Baldwin, private communication). In the March 1985 issue of the Messenger (No. 39, p. 15) M. Rosa pointed out that the response of the lOS system depended on the intensity of the input light source.

ADS BibCode:1985Msngr..41...12NAuthor(s)/Affiliation(s):Noci, G.; Ortolani, S.; Pomilia, A.AA(Istituto di Astronomia, Universita di Firenze) AB(Osservatorio Astronomico di Asiago) AC(Istituto di Astronomia, Universita di Firenze)Abstract:The rotation is a general property of celestial objects, which is probably generated by the vorticity of the interstellar matter. Obviously, stars forming from turbulent vortices conserve some of the initial angular momentum, depending on the early formation history. It is weil known that there is a well-determined trend of the rotational velocity of main sequence and giant stars with the spectral type. This is shown by the continuous and dashed curves of Figure 1, which are from the paper of Bernacca and Perinotto (A A, 33, 443, 1974). Earlytype stars have high rotational velocities, while late-type stars are slow rotators. There is a sharp drop in the velocities from F0 to F5, particularly for main sequence stars: stars later than F5 have all very little angular momentum. This has been attributed to the presence of planets around late-type stars, which would contain most of the angular momentum of the system, as it happens in the case of the solar system. The angular momentum is probably transferred during the T Tauri pre-main-sequence phase. Another possibility suggested for the rotation velocity drop is the loss of angular momentum caused by stellar winds, which should occur for stars having convective layers close to the surface, i. e. late-type stars.

ADS BibCode:1985Msngr..41...16GAuthor(s)/Affiliation(s):Geyer, E. H.; Hoffmann, M.AA(Observatorium Hoher List der Universitäts-Sternwarte Bonn) AB(Astronomisches Institut der UniversitAbstract:Any observer at La Silla who is not working in a telescope control room or watching a movie during a stormy night has the opportunity to see one of the most splendid wonders in the sky without any telescope: Omega Centauri, seemingly a patchy star, but in fact the brightest globular cluster of our Galaxy. Such massive subsystems of a galaxy, each with a content of up to a million stars are quite frequent in the universe. We know many thousands of them, most of them around the giant elliptical galaxies of the Virgo cluster of galaxies.

ADS BibCode:1985Msngr..41...18FAuthor(s)/Affiliation(s):Foing, B. H.; Beckman, J.; Crivellari, L.; Galleguillos, D.AA(European Southern Observatory, Garching, West Germany), AB(Consejo Superior de Investigaciones Cientificas, Instituto de Astrofisica de Canarias, La Laguna, Spain), AC(Osservatorio Astronomico, Trieste, Italy), AD(La Serena, Universidad, Chile; Max-Planck-Institut fuer Radioastronomie, Bonn, West Germany)Abstract:The characteristics of the chromospheres of late-type dwarf stars were studied using the CoudeEchelle Spectrograph (CES) at ESO and the IUE. The study focused on the h and k Mg II lines of F and G dwarfs. The stability of the IUE spectra permitted calculations of averaged absorption features and the subtraction of the Mg II lines to leave intrinsic chromospheric and photospheric line characteristics. Stras with Mg emission cores are very similar in their chromospheres, while stars with Ca II cores vary greatly and exhibit more intense emissions. The higher activity has been linked to magnetic forces which produce spicules with diameters in the range of 1000 km. CES spectra on the Ca II core H lines in Epsilon Eri and Alpha Cen B, active and passive stars, respectively, show a 40 percent lower intensity in the quiescnet star than in the active star, which has an asymmetric core emission and a sharp central absorption with Ca IR triplet lines. The emission features in the active stars are similar to those of the active regions, photospheric spots, chromospheric plages and coronal structures observed in the sun.